clk: renesas: Mark rza2_cpg_clk_register static

This is only used in this file, so mark it static to silence a sparse
warning.

Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>